Configure Hekad poolsize by pillar data

The poolsize must be increased depending on the number of filters.
Typically, the metric_collector on controller nodes and the aggregator on
monitoring node(s) should probably use poolsize=200.
diff --git a/metadata/service/aggregator/single.yml b/metadata/service/aggregator/single.yml
index 9db8b69..7e814c5 100644
--- a/metadata/service/aggregator/single.yml
+++ b/metadata/service/aggregator/single.yml
@@ -3,7 +3,10 @@
 classes:
 - service.heka.support
 parameters:
+  _param:
+    aggregator_poolsize: 100
   heka:
     aggregator:
       enabled: true
       influxdb_time_precision: ms
+      poolsize: ${_param:aggregator_poolsize}